Port GET /user/settings/totp/qrcode to v2 as an image/jpeg blob, modeled in
the OpenAPI spec. Extract the qr-to-jpeg encoding into user.GetTOTPQrCodeAsJpegForUser
so v1 and v2 share it; refactor v1 onto it. The handler reuses the existing
local-account guard, rejecting non-local users with 412.